1. Field of the Invention
The present invention relates to a solid-state image pickup device and in particular to a structure of a metal film which forms wirings and light shielding films of the solid-state image pickup device, i.e. solid state image sensor.
2. Description of the Related Art
Referring now to FIGS. 1(a) and 1(b), there are shown sectional views of a cell portion region (photoelectric conversion portion) and a peripheral region. In the drawing, a reference numeral 1 denotes an n-type semiconductor substrate; 2 a p-type well formed thereon; 3 an n-conductivity type photoelectric conversion region formed in the p-type well 2; 4 an n-conductivity type charge transfer region also formed in the p-type well 2; 5 an element isolation region which is formed between elements by doping a p-type dopant at a high concentration; 6 an insulating film formed on the p-type well 2; 7 an electric charge transfer electrode formed of polysilicon serving also as an electric charge read out electrode; 8 an aluminum wiring formed on the peripheral region; and 8b an aluminum light shielding film having an opening or hole above the photoelectric conversion region 3.
A color filter film 10 and a microlens 11, etc. are formed above the solid-state image pickup device as shown in FIG. 2.
An aluminum thin film is used as the light shielding film in the above mentioned prior art solid-state image pickup device. The aluminum thin film should have a thickness of about 0.8 .mu.m in order to obtain sufficient light shielding ability. The width of the opening of the photoelectric conversion element has been reduced as high density integration of the solid-state image pickup devices has recently been remarkably advanced. A device having 380 thousand pixels and compatible with a 1/3 inch tube has an opening width not higher than 2 .mu.m. It is essential to condense incident light by forming a microlens 11 on the photoelectric conversion element as shown in FIG. 2 in order to prevent lowering of the sensitivity of the device if the opening area is reduced. However, if a conventional aluminum film is used as the light shielding film, the area of the opening formed in the upper side 8ba of the light shielding film 8b would become equal to that in the underside 8bb of the film 8b since the thickness of the aluminum thin film is larger than the dimension of the opening of the light shielding film. That is, the opening would be formed having a narrow width along the total height (usually not less than 1 .mu.m) of the aluminum film including the thickness of the electric charge transfer electrode 7. Accordingly, the incident light from the periphery of the microlens 11 will be reflected in the vicinity of the edge of the opening of the aluminum light shielding film 8b, resulting in lowering the sensitivity of the device.
Regions 14 other than the region 13 to be exposed are exposed due to random reflections of the light on the surface of the aluminum light shielding film 8b as shown in FIG. 3 when a negative photo-resist which will become a dye matrix 12 of the color filter in the solid-state image pickup device using a conventional aluminum light shielding film. As a result of this, patterning accuracy is lowered.
There is the possibility that projections which are referred to as "hillock" may be formed on the aluminum wiring or aluminum light shielding film. This may cause the reliability of the products to be lowered.